The Hornets, without a win in seven ahead of kick-off, used their pace to full effect in the first half to take a 2-0 lead into the break thanks to goals from Troy Deeney and Christian Battocchio.
It appeared as though McCormack's goal was destined to be the winner, with the newly-crowned Sky Bet Championship player of the month pouncing for his 16th of the season and 10th in seven games.
But there was still time for one more twist as Deeney netted with Paddy Kenny on the floor after pushing out George Thorne's shot, giving way to a draw which brought a standing ovation from all four stands.
"I have mixed emotions as it could have gone either way. It was such an open game and in a game like that, anything can happen," McDermott said.
"Even at 3-2 I felt we needed a fourth. We were unfortunate not to get it, but Watford are a good side who were nearly in the Premier League this season.
"We were 2-0 down and I am proud of the players. We were unfortunate not to win but I can't be too greedy.
"There may be an initial disappointment that we didn't win but we may well reflect on this as a good point."
Deeney's second denied Leeds what would have been a sixth home win on the bounce, with McCormack at the fulcrum of that run.
He became the first United player since Jermaine Beckford to score in six successive Elland Road games and McDermott is enjoying watching the Scotland international in action.
"He was bright and vibrant and he makes things happen. We need the big players to come up with the goods," he added.
"He's working so hard, his mentality... He's playing like a leader. He's not at his peak yet and he's getting better all the time. I'm delighted for him and so are the players."
With Leeds in such rich form McDermott could perhaps be happy to shrug off the loss of two points, so Gianfranco Zola's happiness to take a draw was admirable.
His side appear to still be in a throes of a hangover from last season - they lost the play-off final after Leeds beat them on the final day to deny them automatic promotion - and at 2-0 ahead they appeared to be set fair to get back on the winning trail.
"The first half was brilliant, the second... It was a great game, very entertaining and the people who have come here have had a lot of pleasure," the Italian said.
"I am happy about the result of course, a point here at Leeds is not a bad point. I am maybe disappointed that the second goal they scored was a clear foul and I have to say the referee was very good until then.
"In that moment I think he has spoiled the game and the third goal the throw was definitely for us. The mistakes upset me a little bit but at the end of the day I am delighted for a point."
Zola's comments about referee Trevor Kettle were a surprise as few had noticed a foul as Smith scored but after making them, the ex-Chelsea star was keen to stress he was happier with a point than angry at the officials.
"Even after that I say that Leeds deserve a point, they created a lot of chances and a draw is a fair result," he said.
"But we had to show we were not going backwards against Leeds, especially in a stadium like this with a passionate crowd.
"We needed to show spirit and determination and I see a lot of encouraging things happening in my team right now."
- Sports & Recreation
- Troy Deeney
- Brian McDermott
- Ross McCormack